Pulverizers for grinding different types of materials are well known in the prior art. Pulverizers are also known as mills. Solid fossil fuels, such as coal, are one such material wherein there exists a need to grind the material in order to render the solid fossil fuel suitable for use in certain applications, although there are other materials such as gypsum, cement, minerals, etc. that need to be subjected to pulverization as well in order to permit their use in various applications. Fossil fuel fired power generation systems represent one such application in which it is desired to employ pulverized solid fossil fuel, e.g., coal as the fuel. Such a system is commonly referred to as a solid fossil fuel fired system. Pulverized solid fossil fuel firing is favored over other methods of burning solid fossil fuel because pulverized fossil fuel burns like gas and, therefore, fires are easily lighted and controlled.
For purposes of the discussion that follows, the solid fossil fuel fired systems referred to above typically consist of the following major operating components: a solid fossil fuel feeder, an apparatus for pulverizing solid fossil fuel, a distribution system for distributing the pulverized solid fossil fuel, a furnace in which the pulverized solid fossil fuel is to be burned, and the requisite controls for effecting the proper operation of the solid fossil fuel fired system. Of particular interest herein is the apparatus for pulverizing the solid fossil fuel, which will often be referred to hereinbelow as a solid fossil fuel pulverizer. Solid fossil fuel pulverizers are not new. They have been known to exist in the prior art for more than half a century. Furthermore, many improvements in the construction and/or mode of operation of solid fossil fuel pulverizers have been made during this period.
There are a number of features that it is advantageous for any solid fossil fuel pulverizer to possess, but particularly for those which are designed for employment in a solid fossil fuel fired power generation system. Reference is had here to features such as reliability, low power consumption, minimum maintenance and high capacity. In addition, such a solid fossil fuel pulverizer advantageously should also be characterized by quiet operation, integrated lubrication systems, convenient adjustment and control of solid fossil fuel flow and fineness, and the ability to handle the high temperature air that is required for high moisture solid fossil fuel.
One particular type of conventional solid fossil fuel pulverizer is commonly referred to in the industry as an EL type pulverizer. EL type pulverizers are manufactured by the Babcock & Wilcox Company (B&W) and were first produced in the early 1950's. EL type pulverizers have a ball-and-ring design (also known as a ball-and-race design) in which the ball-bearing principle is used to grind the coal. This design uses two vertical axis, horizontal grinding rings and a set of balls placed between the horizontal grinding rings. The lower, or bottom, grinding ring rotates, while the upper, or top, grinding ring remains stationary and is spring loaded to create grinding pressure. The coal is ground by contact with the upper and lower grinding rings and balls (collectively, the grinding elements). The lower and upper grinding rings are each provided with a race having a predefined, matching track contour that engages the balls. The force from the upper grinding ring pushes the balls against the coal layer on the lower grinding ring. The grinding rings and the balls are made of abrasion resistant alloys and comprise the major wear parts of the mill. Ground coal is swept from the grinding zone defined by the grinding rings and the balls by air for final particle size classification and subsequent pneumatic transport to one or more coal burners.
More particularly, FIG. 1 shows a cross section of a B&W EL type pulverizer generally depicted as numeral 100. The pulverizer 100 has a stationary top ring 104, one rotating bottom ring 106, and one set of balls 108 that comprise the grinding elements, as discussed above. The pressure required for efficient grinding is obtained from externally adjustable dual purpose springs 110. The bottom ring 106 is driven by the yoke 112 which is attached to a vertical main shaft assembly 114 of the pulverizer 100. The top ring 104 is held stationary by the dual purpose springs 110. Raw coal is fed into the grinding zone where it mixes with partially ground coal that forms a circulating load. Pulverizer air causes the coal to circulate through the grinding elements where some of it is ground in each pass through the row of balls 108. As the coal becomes fine enough to be picked up by the air it is carried to a classifier 140 where coal of a desired fines is separated from the stream entering the classifier 140 and is carried out with the air. Oversized material is returned to the grinding zone. The pulverizer 100 is driven by a bevel gear assembly 141 positioned on the vertical main shaft 114 in engagement with a pinion shaft gear 142 positioned on the horizontal pinion shaft 116. The bevel gear assembly 141 comprises a bevel gear 143, which is mounted to a gear center 144 that is, in turn, mounted to the vertical main shaft 114.
FIG. 2 is a more detailed view of the main shaft 114. Recessed into the main shaft 114 is a yoke keyway 201A and a bevel gear keyway 203A. The yoke keyway 201A is associated with a yoke key 201B, and the bevel gear keyway 203A is associated with a bevel gear key 203B, which is also often referred to as a center gear key. The yoke key 201B fits into the yoke keyway 201A and a corresponding slot (not shown in the Figures) in the yoke 112 when the yoke 112 is mated to the main shaft 114. The yoke key 201B, when installed, transfers drive torque from the rotating main shaft 114 to the yoke 112. The bevel gear key 203B fits into the bevel gear keyway 203A and a corresponding slot (not shown in the Figures) in the bevel gear assembly 141 when the bevel gear assembly 141 is mated to the main shaft 114. The bevel gear key 203B, when installed, transfers drive torque from the rotating horizontal pinion shaft 116 to the main shaft 114. The yoke 112 is secured to the main shaft 114 via yoke locknut 136A, and the bevel gear assembly 141 is secured to the main shaft. 114 via gear locknut 136B.
Failure of the main shaft 114 occurs most commonly at one of the keyways 201A or 203A. This is because the material removed from the main shaft 114 to form a keyway creates a stress concentration point that weakens the main shaft 114. The stresses that initiate main shaft 114 failure in EL type pulverizers primarily come from the energy of the several balls 108 which roll along in between the upper 104 and lower 106 grinding rings and also rotate about the vertical axis of the main shaft 114, and the reaction forces of the spring loaded top grinding ring 104 to the balls 108. It has been determined that eccentric loadings on the main shaft 114 caused by the movements of these balls is a primary cause of such main shaft failures. When a main shaft 114 fails, expensive and time consuming repairs must be undertaken, including replacing the failed main shaft 114.
Accordingly, a need exists for an improved main shaft that is not subject to keyway failure.
Furthermore, because of the use of keys, assembly of the main shaft 114 and its components is a complicated process. When installing the bevel gear assembly 141 on the main shaft 114 the slot formed in the bevel gear assembly 141 must be aligned with the bevel gear keyway 203A so that the bevel gear assembly 141 can be seated over the bevel gear key 203B in the bevel gear keyway 203A. Likewise, when installing the yoke 112 on the main shaft 114 the slot formed in the yoke 112 must be aligned with the yoke keyway 203A so that the yoke 112 can be seated over the yoke key 201B in the yoke keyway 201A. The yoke 112 and bevel gear assembly 141 aligning processes are both time consuming and difficult tasks.
Also complicating the process of assembling the main shaft 114, once the bevel gear assembly 141 has been properly seated, the gear locknut 136B must be installed. Also, once the yoke 112 has been properly seated, the yoke locknut 136A must be installed. Locknut installation is a tedious and time consuming job, involving heating the yoke 112 or bevel gear assembly 141, and hammering the yoke locknut 136A or gear locknut 136B. Accordingly, a need exist for an improved technique to assemble a main shaft and its components.